1. Field of the Invention
This invention relates to frame structures for communication systems and more particularly to frame structures for adaptive modulation wireless communication systems.
2. Description of Related Art
A wireless communication system facilitates two-way communication between a plurality of customer premises equipment (“CPE”) and a network infrastructure. Exemplary systems include mobile cellular telephone systems, personal communication systems (PCS), and cordless telephones. The objective of these wireless communication systems is to provide communication channels on demand between the users connected to a CPE and a base station in order to connect the user of a CPE with a network infrastructure (usually a wired-line system). In multiple access wireless schemes, the basic transmission unit is commonly frames of time. The frames are commonly divided into a plurality of time slots. The time slots of the frames may hold different kinds of data including control data and user information or data. In order to manage the use of the time slots of a frame, the time slots may be assigned or allocated to one or more CPEs. In this case, a CPE receiving or having an allocation of time slots may parse the allocation of the slots between one or more users associated with the CPE. CPEs typically communicate with a base station using a “duplexing” scheme that allows for the exchange of information in both directions of connection. In this scheme, the time slots of each frame may be allocated to data being transmitted from a base station to CPEs and to data being transmitted from CPEs to a base station.
Transmissions from a base station to a CPE are commonly referred to as “downlink” transmissions. Transmissions from a CPE to a base station are commonly referred to as “uplink” transmissions. Prior art wireless communication systems typically employ time division duplexing (TDD) to facilitate the exchange of information between base stations and CPEs where TDD is well known in the art. In TDD systems, duplexing of transmissions between a base station and associated CPEs is performed in the time domain. Further, the CPEs typically communicate with their associated base station with signals having a specific pre-defined radio frequency. In TDD systems, the bandwidth or channel of the signal is time-divided into frames having repetitive time periods or time “slots”. The time slots are employed for uplink and downlink transmissions between the base station and associated CPEs.
When a wireless system is implemented in a region, the region is commonly divided into cells with a base station located within each cell. Each base station in a cell of the wireless system ideally provides communication between CPEs located in the cell. The size or configuration of a cell is generally determined as a function of the physical location of the base station, the location of buildings and other physical obstructions within the cell. The maximum bit per symbol rate modulation scheme that may be employed with a cell may be limited due to channel interference and the implementation or modem complexity of CPEs within the cell. Channel interference may occur between adjacent time slots assigned to different CPEs within a cell due to distortion of signals between the base station in the cell and the CPEs. The signals are commonly distorted by destructive multi-path replication of the signals (where the signals are reflected off physical objects in the cell). In addition, the signals are commonly distorted by atmospheric conditions (such as rain). Thus, in order to have duplex communications between all CPEs associated with a base station in a cell, a modulation scheme having a bit per symbol rate that enables communication between all CPEs associated with the base station is selected.
It is noted, however, that the channel interference between CPEs and a base station varies for each CPE, e.g., as a function of the physical barriers between the base station and the CPE. Consequently, the maximum bit per symbol rate modulation scheme (i.e., having acceptable error rates given the channel interference) that may be used to communicate between each CPE and the base station may vary. In addition, the implementation or modem complexity of the CPEs associated with the base station may also vary where some CPEs may be able to support higher bit per symbol rate modulation schemes than others associated with the base station. Accordingly, the selection of one low bit per symbol rate modulation scheme for all CPEs where some CPEs may support a higher bit per symbol rate modulation in a cell may not maximize bandwidth utilization. The use of different or variable bit per symbol rate modulation schemes for different CPEs associated with a cell may increase bandwidth utilization. Unfortunately, variable bit per symbol rate modulation is not used for communication between base stations and associated CPEs due to its complexity. In particular, variable bit per symbol rate modulation schemes normally require complex CPE demodulators where some CPEs may already have limited implementation or modem complexity. The need thus exists for frame structures and frame construction techniques that enable variable bit per symbol rate modulation for CPEs and base stations within a cell that does not increase the complexity of CPEs. The present invention provides such a frame structure and frame construction techniques.